The present invention relates to tobacco expansion and more particularly to the preservation of high heat transfer atmospheres in tobacco expansion chambers.
In tobacco expansion processes, it is common to initially impregnate tobacco with an expansion agent such as carbon dioxide or inert oganic liquids. Subsequent to impregnation, the tobacco is subjected to a stream of hot gases, typically steam, air, etc. thereby vaporizing liquid impregnants and subliming solid impregnants. The vapor phase of the impregnant is formed at a greater rate than the rate at which it escapes from the intercellular spaces in the tobacco and consequently the tobacco is blown up in size, i.e. expanded, from within. The application of positive heat to the impregnated tobacco will accelerate expansion as the rate of sublimation of a solid CO.sub.2 impregnant, for example, will be more rapid. A process for so expanding tobacco with liquid CO.sub.2 utilized as an impregnant is disclosed in U.S. Ser. No. 439,804, filed Feb. 5, 1974 and assigned to the assignee of the present invention. In this process, hot air is typically utilized as the expansion medium. U.S. Pat. Nos. 3,978,867 and 4,069,830 also disclose tobacco expansion processes utilizing hot air as the expansion medium.
The use of hot air as an expansion medium has its drawbacks, primarily the fact that air has a relatively low thermal diffusivity. As the degree of tobacco expansion is partially dependent upon the rate at which the impregnated tobacco is heated, it is desirable to retain a chamber atmosphere of a composition which is effective for heat transfer. Air alone is not the most effective expansion medium and compensation for such non-optimal atmospheres by raising atmosphere temperature is undesirable as excessive temperatures will result in scorching or other damage to the tobacco. As illustrated in U.S. Pat. Nos. 3,524,452 and 3,753,440, it has been proposed to improve heat transfer from a gaseous expansion medium such as freon and air by adding steam to the expansion chamber. While steam additions generally improve the rate at which tobacco can be expanded, steam generating equipment is costly and requires considerable quantities of energy for its operation. Thus, although steam exhibits a relatively high thermal diffusivity, the cost of producing steam may outweigh gains in expansion performance, e.g. about a 10% increase in expansion. It is also known to utilize steam heat in freeze-dry tobacco expansion processes, a typical one of such processes being illustrated in U.S. Pat. No. 3,991,772.
In automated tobacco expansion systems, impregnated tobacco is commonly expanded by contact with a stream of heated gases in an expansion tower and then passed with such gases to a separation device such as a cyclone separator. As illustrated in U.S. Pat. No. 3,524,452, gas phase effluent of the separation device is reheated and returned to the expansion tower. Tobacco is metered into the expansion tower and discharged from the separation device by means of mechanical solids feeding devices such as starwheel valves. Although the use of such valves enables continuous flows of tobacco to be sustained, these valves are relatively ineffective in precluding gas flows therethrough. Consequently, air is readily admitted into the expansion tower and separation device and becomes a significant component of the expansion medium, i.e. internal expansion chamber atmosphere. As the thermal diffusivity of air is relatively low in comparison to carbon dioxide or steam, the latter must frequently be added to expansion chambers to enable atmospheres of adequate thermal diffusivities to be achieved.
Accordingly, there is a need to efficiently expand tobacco impregnated with an expansion agent such as carbon dioxide without the use of steam and without resort to excessive temperatures in an expansion chamber.